For production of a fluoropolymer such as polytetrafluoroethylene (hereinafter referred to as PTFE) as a non-melt processible fluororesin, a melt processible fluororesin or a fluorinated elastomer, aqueous emulsion polymerization is applied. In the aqueous emulsion polymerization of a fluoromonomer, in order that the polymerization reaction may not be inhibited by chain transfer, as an emulsifier, ammonium perfluorooctanoate (hereinafter referred to as APFO) which is a fluorinated emulsifier has been commonly used. Further, also in suspension polymerization of PTFE, such a fluorinated emulsifier is added in an extremely small amount at the time of polymerization for the purpose of improving powder properties of a product. However, high bioaccumulation potential of APFO has been pointed out, and disposal of hardly decomposable APFO has been considered to be environmentally problematic, and thus a fluorinated emulsifier which replaces APFO has been required. In recent years, it has been proposed that an ammonium salt of a fluorinated carboxylic acid having an etheric oxygen atom is effective as a fluorinated emulsifier having low bioaccumulation potential as compared with APFO, and its application to aqueous emulsion polymerization and suspension polymerization is in progress (the following Patent Documents 1 and 2). Further, the following Patent Document 3 reports a novel method for producing a fluorinated carboxylate having an etheric oxygen atom.
The ammonium salt of a fluorinated carboxylic acid having an etheric oxygen atom is contained, like APFO, in a waste liquid after an aqueous emulsion of a fluoropolymer is coagulated and the fluoropolymer is separated, after used for the production of the fluoropolymer, and an exhaust gas in a step of drying and/or a step of heat treatment of the separated fluoropolymer, and is preferably recovered and reused.
Further, the ammonium salt of a fluorinated carboxylic acid having an etheric oxygen atom is contained in a liquid obtained by cleaning an anion exchange resin which has been brought into contact with the waste liquid, with an alkaline aqueous solution. Further, the ammonium salt of a fluorinated carboxylic acid having an etheric oxygen atom is contained in a liquid obtained by cleaning an anion exchange resin which has been brought into contact with an aqueous dispersion obtained by adding a nonionic surfactant to the aqueous emulsion of the fluoropolymer, with an alkaline aqueous solution. It is desired to recover and reuse the ammonium salt of a fluorinated carboxylic acid having an etheric oxygen atom also from these liquids.
Heretofore, to recover APFO used for the production of a fluoropolymer, a method of adding an acid to APFO in the aqueous medium to liberate perfluorooctanoic acid (hereinafter referred to as PFOA), and reacting it with an alcohol to form an ester, which is obtained by distillation, a method of acidifying an aqueous solution containing APFO with an acid, followed by heating and separation into two layers, and subjecting the obtained organic liquid to steam distillation to recover PFOA, a method of acidifying a sodium salt of PFOA, decomposing concomitants and impurities with an oxidizing agent, followed by distillation to recover PFOA, etc., have been known (the following Patent Documents 4, 5 and 6).
However, if the conventional method effective for APFO is applied to recovery of a fluorinated carboxylic acid having an etheric oxygen atom as it is, the obtained fluorinated carboxylic acid having an etheric oxygen atom contains a large amount of organic impurities. The organic impurities are difficult to remove after conversion to a fluorinated carboxylate and have influences over the surface tension lowering properties of the fluorinated carboxylate, and if such a fluorinated carboxylate is used as an emulsifier for a fluoromonomer, there are such problems that the polymerization stability is insufficient, an obtainable fluoropolymer will be colored, and the quality will be decreased.
Patent Document 1: WO2007/046345
Patent Document 2: US2007/0015864
Patent Document 3: JP-A-2006-321797
Patent Document 4: JP-A-7-53465
Patent Document 5: JP-A-2004-506708
Patent Document 6: JP-A-6-25072